A more thorough explanation:

The CAN-SPAM Act of 2003 is a law that limits the transmission of spam emails, including those containing pornographic material. Congress enacted this law to prevent spammers from using misleading subject lines and fraudulent methods to distribute pornographic content.

According to the Senate Commerce Committee Report, pornographic spam is more likely to contain misleading subject lines than other forms of spam. The Committee also disapproved of spammers' methods of enabling pornographic images to open with the opening of a spam email.

Section 5(d) of the CAN-SPAM Act of 2003 addresses concerns related to emails containing sexually oriented material. This section requires clearly identifiable marks and notices to alert recipients to an email's pornographic content. It also requires technical measures to ensure that additional steps must be taken after opening an email before a pornographic image may be viewed.

For example, if a spam email contains pornographic material, it must have a clear warning in the subject line or body of the email. Recipients must also take additional steps, such as clicking a link or entering a password, before they can view the pornographic content. These measures help prevent minors and others who do not wish to view such content from accidentally accessing it.